This invention relates to fluid filters, and more particularly to automatic transmission fluid filters which require a minimum flow rate at low operating temperatures.
Automatic transmissions require a filter to remove harmful materials from the fluid being circulated through the transmission system. Various configurations of filters have been suggested for use in automatic transmission fluid circulating systems. One such filter is disclosed in U.S. Pat. No. 4,402,827 to Joseph. This filter is made from two generally rectangular cup-shaped pan members having flanged peripheries fastened together in opposed relation. A filter element envelope is made in part from a long piece of resin-impregnated felt folded over double, and in part from a smaller piece of nylon mesh that replaces an area of the felt material in a window-like fashion. The edges of the filter element are captured between the fastened flange sections of the pan members.
In filters of the disclosed design, a pump creates suction which draws fluid from a sump, into the filter, through the walls of the envelope and through the outlet of the filter. Because of this suction, the pressure on the inside of the envelope is less than the pressure on the outside, and raised bosses are typically provided on the pan members to support the filter media to prevent it from contacting the walls of the pan members.
During testing of filters constructed similar to this fashion, it was found that insufficient flow rates were produced during low temperature operations. Increasing the surface area and porosity of the filter media were considered to improve the situation, but were undesirable from the standpoint of filter size or filter inefficiency.
It was discovered that placing a grid inside the filter envelope to maintain the spacing between the sheets of the envelope improved the flow rate. Thus it appeared that, contrary to expected behavior, the filter envelope was collapsing during low temperature operation, and a spacer means solved the problem.